It is known that organic polymers each containing at least one reactive silyl group per molecule are characterized by being cross-linked by the siloxane bond formation involving the hydrolysis or other reactions of the reactive silyl group due to moisture or other factors even at room temperature, whereby a rubbery cured product is formed.
Among such reactive silyl group-containing organic polymers, those whose backbone is a polyoxyalkylene polymer, a saturated hydrocarbon polymer, or a poly(meth)acrylate copolymer are disclosed in, for example, Patent Literature 1, have already been produced industrially, and are widely used in applications such as sealants, adhesives, coating materials, and paints.
Especially those having a methyldimethoxysilyl group as the terminal reactive silyl group and having a polyoxyalkylene polymer as the backbone are widely used for sealants for buildings because they have low viscosity and good moisture permeability, adhesion, and weather resistance, and the like (Non Patent Literature 1, pp. 156-157). Sealants for buildings may be required to give a low-modulus cured product capable of being highly stretched by a small amount of stress. Low modulus may be achieved by, for example, a method of using a linear polymer with a higher molecular weight such that the molecular weight between crosslinking points is increased, or a method of adding a large amount of a plasticizer. Also, sealants for buildings may be required to be in a one-component form which eliminates the need for mixing ingredients before application and thus has excellent workability. In the case of preparing a one-component sealant using a reactive silyl group-containing organic polymer, a tetravalent tin compound stable as a curing catalyst is typically used.
When a low-modulus sealant containing a tetravalent tin compound as a catalyst is used for a long period of time, especially in a joint that will greatly shrink, the sealant has unfortunately been found to have wrinkles or cracks on the surface.
These defects are presumed to be caused by the use of a tetravalent tin compound as a catalyst which reduces the recovery of the cured product so that, in the case of using the cured product in a joint that will greatly shrink, the product cannot follow the shrinkage and eventually has wrinkles or cracks, as described in Patent Literature 2 (p. 34, lines 7-6 from the bottom). Low-modulus sealants have also been found to give cured products with lower recovery to than high-modulus sealants, which is considered to be another factor causing wrinkles or cracks.
Although it has been found that wrinkles can be prevented by the use of a high-modulus sealant obtained by reducing the molecular weight between crosslinking points in the polymer or by reducing the amount of plasticizer, such a method cannot be employed in cases requiring low modulus. It has also been found that wrinkles can also be prevented by the use of a divalent tin compound or a compound other than tin instead of the tetravalent tin compound as a catalyst, but it is not always easy to achieve the same properties (e.g. curing rate, storage stability) as those of one-component sealants containing a tetravalent tin compound.